Fujioka Haruhi - Horikita Maki (everyone seems to like her as Haruhi and I can't think of anyone better)

Suou Tamaki - Matsumoto Jun (my 1st choice was Teppei Koike, but hes too short I think for this role. Perhaps the anime seiyuu could also play this role as someone mentioned before. I haven't seen him act though.)
Now that I think about it though, Ikuta Toma might play Tamaki pretty well. He's hilarious, so I think itd work out.

Actually it all started as a small rumors about years ago... until this year it was confirm that they are gonna make a live version I heard from a classmate of mine... to be frank... I am not sure whether they are gonna Ouran a live action...